In a modern internal combustion engine of the common-rail type, a high pressure pump receives a fuel flow from a reservoir by means of a low pressure pump and supplies the fuel to a common rail which is hydraulically connected to a plurality of injectors. The pressure of the fuel within the common rail must be constantly controlled as a function of the status of the engine by varying the instantaneous flow rate of the high pressure pump or by always supplying an excess of fuel to the common rail and discharging from the common rail itself the excess fuel by means of a control valve. Generally, the solution of varying the instantaneous flow rate of the high pressure pump is preferred, because it displays an energy efficiency which is definitely higher and does not imply an overheating of the fuel.
A solution of the type set forth in patent application EP1612402A1 has been suggested to vary the instantaneous flow rate of the high pressure pump, the application relating to a high pressure pump comprising a number of pumping elements reciprocatingly actuated through corresponding suction and discharge strokes and in which each pumping element is provided with a corresponding suction valve in communication with a suction conduit supplied by a low pressure pump; a slide valve is arranged on the suction conduit, the slide valve being chopper controlled synchronously with an initial part of the suction step of each pumping element. In other terms, the slide valve is a valve of the open/closed type (on/off type) which is driven by modifying the ratio between the opening and closing intervals to vary the instantaneous flow rate of the high pressure pump. In this manner, the slide pump always displays a wide effective passage section that does not determine a significant loss of local pressure (loss of local load).
In patent application EP06425612.6 a slide valve for the flow rate of a fuel pump has been suggested, which is provided with: a cylindrical tubular valve body, which is closed at the top, displays a cylindrical seat which in its lower portion serves as a conduit for the fuel, and comprises a number of radial through bores to allow the entry of the fuel within the cylindrical seat; a lower disk, which is arranged within the cylindrical tubular valve body below the radial bores and displays a central through bore which defines an outlet opening for the fuel; and a cylindrical obturator, which is coupled to the lower disk and is mobile between an open position, in which the outlet opening is in communication with the radial bores, and a closed position, in which the outlet opening is isolated from the radial bores.
An electromagnetic actuator is provided to shift the obturator from the closed position to the open position against the bias of a spring. The electromagnetic actuator comprises a coil arranged externally around the tubular valve body, a fixed magnetic pole, which is arranged within the tubular valve body, a mobile keeper, which is mechanically connected to the obturator and is adapted to be magnetically attracted by the magnetic pole when the coil is excited, a tubular magnetic armature, which is arranged outside the tubular valve body and comprises an annular seat to house the coil therein, and an annular magnetic washer, which is arranged above the coil to guide the closing of the magnetic flow around the coil itself.
The coil is maintained in position by the tubular magnetic armature and by the washer, which are locked against the tubular valve body by means of an interference driving. However, the interference driving of the tubular magnetic armature occurs in an area of the tubular valve body arranged near the mobile keeper; accordingly, by the effect of the interference driving of the tubular magnetic armature, the tubular valve body could locally be subjected to deformations modifying the stroke of the mobile keeper and thus modifying in an unacceptable manner the performance of the slide valve. Specifically, it has been observed that to carry out the interference driving of the tubular magnetic armature on the tubular valve body it is required to hold the tubular magnetic armature still and axially thrust on the tubular valve body; such an axial thrust on the tubular valve body is especially negative because it may easily determine localised deformations of the tubular body.
The locking of the tubular magnetic armature to the tubular valve body by welding has been suggested in order to attempt to solve the above described drawback; however, the execution of the welding considerably increases the assembly costs of the slide valve and further causes a localised retraction of the material that determines a modification of the stroke of the mobile keeper.